Many modern ultrasound systems employ phased array transducers which comprise a number of individual transducers that are arranged, side by side, in a unitary assembly. In effect, the phased array of transducers acts like an acoustic lens with an electronically variable focal length. Such a phased array transducer, while stationary, is electronically controllable to steer and focus an ultrasound beam so as to achieve a B scan image.
B or brightness mode scanning is the most common type of ultrasound imaging. A B scan is a view of a cross-sectional slice through the object being imaged. A narrow pencil beam of ultrasound energy is swept through a scan sector to define a scan plane. The beam is formed from bursts of ultrasound and the repetition rate of the ultrasound pulse generation is selected so that the transmitted pulse has time to travel to the deepest target and back again before the next pulse is launched. The echoes vary in intensity according to the type of tissue or body structure causing the echoes. The echo return data is presented on a display in which the brightness depends upon the echo strength.
In addition to a phased array being able to provide an image in a single scan plane, certain phased array transducers are rotatable about an axis that is orthogonal to the transducer's emitting face. The rotation of the phased array structure enables the scan plane to be rotated about the orthogonal axis and thus enables multiple images to be acquired at various rotated scan plane orientations.
In addition to phased array ultrasound transducers, other types of ultrasound probes also are adapted to generate multiple image planes, I. e., "wobbler" probes, tomographic-type probes which produce multiple parallel planes at known spatial intervals, etc. Each of the abovementioned probes enables acquisition of more than one, two-dimensional ultrasound image without requiring a physical movement of the entire probe housing.
Ultrasound scanners typically display a two dimensional B scan image as a sector (or other geometrical shape) on a display screen. Although the image data within the sector changes as the transducer is moved and different areas of tissue are scanned, the sector shape, itself, remains constant regardless of transducer orientation. Further, when the scan plane of a rotatable phased array ultrasound probe is changed, the prior art display presents an identical sector scan as for all other scan planes.
A further indicator is sometimes provided on the display which informs the operator of the angular sensor position with respect to the tip of the ultrasound transducer. Such indicator may be an angle icon. The user is then forced to conceptualize the orientation of the scan plane in three-dimensional space and, in particular, in a three-dimensional position that is relative to other image planes acquired at the same probe tip position, but using different sensor angles. Such visualization on the part of the user requires substantial training and is still subject to error if, for any reason, the user is distracted during the ultrasound examination.
Thus, there is a need for an improved imaging method for multiplanar ultrasound images which enables the user to better visualize three dimensional objects that are scanned using multiple non-coplanar scan planes. There is further need for an improved ultrasound imaging method which enables a quasi-three dimensional viewing of scan planes that are non-coincident with a reference scan plane.